1. Field of the Invention
This invention relates to a method, apparatus and lead frame for assembling leads with terminals on a substrate, and more particularly to a method, apparatus and lead frame for assembling leads with terminals on a substrate in which removable substrate aligning tabs are formed integrally with each lead frame in a continuous strip of lead frames and in which the lead frame strip transports the substrates through a series of processing stations.
2. Description of the Prior Art
It is standard practice in the manufacture of electronic equipment, such as thin film circuit devices, to bond leads of a lead frame to terminals or bonding pads on a ceramic substrate in an operator attended position. In a known bonding operation, which utilizes individual separated lead frames, the operator initially positions the ceramic substrate into a nest with the edges of the substrate adjacent respective small rigid metal (e.g., steel) alignment pins embedded in a support block for locating the substrate in a desired position. Next, a lead frame is placed by the operator over the ceramic substrate and inner free end portions of the leads of the lead frame are manually aligned with the terminals on the substrate. A thermode of a substrate bonding tool then is brought downward into contact with the inner lead end portions to thermocompression bond them to the terminals. Subsequently, the resultant bonded lead frame-substrate assembly is manually removed from the bonding apparatus and the process is repeated.
The foregoing arrangement is undesirable in that it is relatively time consuming, laborious and subject to human error. The leads of the lead frame also are subject to contamination, bending or other damage as a result of the manual handling thereof. In addition, it is difficult to preheat the separate lead frames to facilitate the bonding of the substrates thereto, and thus the bonding apparatus must be operated at a high power level to achieve satisfactory bonds.
The above-described system also is disadvantageous in that the fixturing for receiving and aligning the ceramic substrates is expensive to fabricate, since the small metal alignment pins must be located therein with a high degree of accuracy. In this connection, however, the fixturing still is relatively inaccurate because the alignment pins cannot be located in spaced relationship exactly corresponding to the dimensions of the substrates, but must be located distances apart somewhat greater than the substrate dimensions to provide clearance between the pins and edges of a substrate positioned therein. Otherwise, the expansion of the substrate against the rigid alignment pins as the substrate absorbs heat during the bonding operation, could cause cracking of the substrate. Further, it has been found that the metal alignment pins tend to become worn by the edges of the ceramic substrate, which are relatively hard and abrasive in nature, such that the pins soon are no longer capable of locating one of the substrates in proper position below the thermode of the bonding tool with any degree of accuracy, whereby the pins must be frequently replaced.
In an attempt to increase production output, the use of fixturing having a plurality of substrate receiving and aligning positions, for processing a short length of a lead frame strip (e.g., 5 lead frames long), also has been utilized. Initially, the substrates are manually positioned into nests between respective sets of rigid metal alignment pins and the lead frame strip is manually located on the fixturing with leads of each lead frame aligned with bonding pads or terminals on respective ones of the substrates. The fixturing then is indexed into a bonding tool which bonds each of the lead frames to their respective substrates in sequence, after which the strip of bonded lead frames and substrates is manually removed from the apparatus and the process is repeated. This arrangement, however, still is subject to the other disadvantages noted above in connection with the use of separate lead frames, and the fixturing is considerably more expensive because of the multiple sets of accurately located alignment pins which are required.